Potential Vorticity Dynamics of Tropical Instability Vortices

What is it about?

In this article we examined the dynamics of tropical instability vortices from a vorticity and potential vorticity perspective. Tropical instability vortices form on the northern edge of the eastern equatorial Pacific cold tongue and play an important role in the large-scale momentum and heat budgets of the equatorial region. Here we use a novel lagrangian vorticity dynamics approach to examine how the strong anticyclonic rotation within the vortex cores is generated. By constructing budgets of vertical vorticity along virtual Lagrangian fluid parcel tracks entering the vortex, we show that a significant proportion of their vorticity is sourced from the horizontal vorticity associated with the vertical shear of the Equatorial Undercurrent (EUC). Furthermore, we show that this process involves the conversion of baroclinically-low potential vorticity fluid (i.e. low PV due to a strong horizontal buoyancy gradient and vertical shear) to vortically-low potential vorticity (i.e. low PV due to strong anticyclonic rotation) through vortex titling, a process also common in the formation of submesoscale coherent vortices at midlatitude submesoscale fronts.

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Why is it important?

In this article we examine the dynamics of tropical instability vortices from the perspective of their vorticity dynamics, as opposed to the more typical energetics approach. This alternative approach yields additional insight into the processes that are involved in the formation of these vortices. In particular, we highlight the similarities between the dynamics of these large-scale (~500km lateral scale) equatorial flows and the submesoscale regime (~10km lateral scale) in the midlatitudes. Such similarities could potentially be exploited to better understand how these vortices operate, and what role they may play in the larger scale circulation. It also highlights the benefits of thinking about dynamical regimes as opposed to specific scales.